personally i wouldn't go with anything under 3000w when I started I was using a 2400 numatic vacuum.
I was more than happy with it i used it for about a year but when i upgraded to the 3000w what a difference.
If I was looking to buy a vacuum for gutter cleaning now you couldn't even give me my old 2400w machine i wouldn't want it. the time saved having the extra power is amazing.
I done a block of flats last year 5 hours it took me, I done the same block of flats this year 1 hour and 15 minutes, still charged the same amount.
having the extra power means your not constantly going over the same area poking at it trying to suck it all up.